1. Technical Field
The present invention generally relates to wavelength division multiplexing systems and, more particularly, to an improved grating configuration for a phasar.
2. Technical Background
The simultaneous transmission of several channels along a single path is known as wavelength division multiplexing. This technology is important in fiber based systems due to its impact on system configuration, performance and cost. One major advantage of this technology is its ability to increase system capacity by increasing the number of channels that can be carried per fiber. With the ability to increase capacity, existing systems can be upgraded rather than replaced.
In wavelength division multiplexing systems, each channel operates at a different wavelength. The individual wavelengths are combined into a single path by a multiplexer and are separated from one another by a demultiplexer. Multiplexers and demultiplexers normally take the form of a dispersive element such as a diffraction grating, prism, or hologram. When using a grating, demultiplexing is accomplished by transmitting the multiplexed signal through the grating which separates the individual wavelengths from one another and diffracts each in a slightly different direction. Multiplexing is accomplished by utilizing the same device in reverse.
As wavelength division multiplexing technology has evolved, the need for more complex photonic components such as gain-flattening filters, variable attenuators, and add-drop multiplexers has increased. An efficient way to package such components is to use a planar photonic device. Planar photonic devices are chosen for their ability to implement many optical functions on a common wafer. In addition, active devices can be added to the wafer in order to create hybrid packages delivering many of the functions (e.g., switching, attenuating, monitoring, multiplexing and demultiplexing, etc.) needed in optical networks in very compact packages.
When combining many different elements on the same wafer, noise or cross-talk generated by each element sometimes interferes with the other elements. This greatly decreases the performance of the overall system. Cross-talk is an especially critical consideration in phasars for dense wavelength division multiplexing applications where more and more signals with different wavelengths travel through the network. Each wavelength must be correctly routed to the correct detector or fiber, and must not interfere with the other wavelengths.
Other requirements for phasars have also increased dramatically in the last decade. In addition to having less cross-talk, phasars must have better insertion losses, work with a greater number of channels, and have denser spectral spacing. These specifications have resulted in the development of larger and larger devices. Since cross-talk is dependent on the length traveled in phasars, such larger devices are more sensitive to cross-talk. In the case of refractive index, core layer, or etching non-uniformity in the wafer, the accumulated phase error along each grating waveguide increases dramatically with length.
Conventional gratings for phasars include a plurality of adjacently arranged waveguides forming an array. The array of waveguides includes a first portion extending in a straight direction, a second portion bending in a clockwise direction, and a third portion extending in a straight direction. The second, clockwise bend portion has a fixed or variable radius of curvature bringing the direction of the array to the horizontal. This array is then repeated in a mirror image for the opposite side of the phasar.
Although such conventional grating configurations have achieved great success, there is room for improvement in the art. For example, the first straight portion of the grating forces wavelengths to travel a longer distance than is necessary thereby increasing the potential for cross-talk interference. As such, it would be desirable to provide a compact grating configuration for reducing cross-talk interference in wavelength division multiplexing systems by decreasing the distances that wavelengths must travel.
The above and other objects are provided by a grating having a generally s-shaped waveguide configuration. The grating includes a plurality of waveguides arranged adjacent to one another to form an array. The array includes a first portion bending in a counter-clockwise direction, a second portion bending in a clockwise direction, and a third portion extending in a straight direction. The bends have either a fixed or a variable radius of curvature. The arc length of each waveguide along the counter-clockwise bend portion varies from one waveguide to the next. The arc length of the first counter-clockwise bend is nearly zero. The different arc lengths along the counter-clockwise bend cause some waveguides to be longer than others. This results in a clear separation of the individual waveguides. Further, the overall dimension of the grating is smaller than conventional devices which leads to reduced cross-talk.